The present invention relates to printing fluid input systems for use in keyless lithographic printing processes.
In the field of high speed lithographic printing, ink is continuously conveyed from an ink source by means of a series of rollers to planographic printing plate on a plate cylinder in a lithographic printing press. Image portions of the printing plate accept ink from one or more of the last of a series of inking rollers and transfer a portion of that ink to a blanket cylinder as a reverse image from which a portion of the ink is transferred to form a correct-reading image on paper or other materials. It is also essential in conventional lithographic printing processes that a dampening solution containing water and proprietary additives be conveyed continuously to the printing plate whereby transferring in part to the non-image areas of the printing plate the water functions to keep those non-image areas free of ink. Hereinafter, the terms "water" and "dampening solution" refer to water plus additives or to other aqueous solutions used in the operation of lithographic printing presses.
In conventional printing press systems, the ink is continuously made available in varying amounts determined by cross-press column input control adjustments to all parts of the printing plate, including both image and non-image areas. In the absence of the dampening solution, the printing plate will accept ink in both the image and non-image areas of its surface.
Lithographic printing plate surfaces in the absence of imaging materials have minute interstices and a hydrophilic or water-loving property to enhance retention of water, that is the dampening solution, rather than ink on the surface of the plate. Imaging the plate creates oleophilic or ink-loving areas according to the image that is to be printed. Consequently, when both ink and dampening solution are presented to an imaged plate in appropriate amounts, only the ink tending to reside in non-image areas becomes disbonded from the plate. In general, this action accounts for the continuous ink and dampening solution differentiation on the printing plate surface, which is essential and integral to the lithographic printing process.
Controlling the correct amount of dampening solution supplied during lithographic printing has been an industry-wide problem ever since the advent of lithography. It requires continual operator attention since each column adjustment of ink input may require a change in dampener input. Balancing the ink input that varies for each column across the width of the press with more or less uniform dampening solution input across the width of the press is at best a compromise. Consequently, depending upon which portion of the image the operator has adopted a his standard of print quality at any given time during the printing run, the operator may need to adjust the ink input at correspondingly-located cross-press positions. As a result, the dampening solution to ink ratio at that position ma become changed from a desired value. Conversely, the operator may adjust the dampener input for best ink and dampening solution balance at one inking column, which may adversely affect the ink and dampening solution balance at one or more other cross-press locations. Adjustments such as these tend to occur repeatedly throughout the whole press run, resulting in slight to significant differences in the quality of the printed image throughout the run. In carrying out these adjustment operations, the resulting images may or may not be commercially acceptable, leading to waste in manpower, materials, and printing machine time.
Certain commercially successful newspaper printing configurations rely on the inking train of rollers to carry dampening solution to the printing plate. Notable among these are the Goss Metro, Goss Metroliner, and the Goss Headliner Offset printing presses which are manufactured by the Graphic Systems Division of Rockwell International Corporation. In these alternative configurations, the dampening solution is combined with the ink on an inking oscillator drum such that both ink and water are subsequently and continuously transferred to the inking form rollers for deposition onto the printing plate. In another variation, the dampening solution is applied in a conventional manner directly to the printing plate by means of separate dampening rollers and a dampening solution supply system. In systems of either type, regardless of the method whereby the dampening solution is introduced, it is well known that some of the dampening solution becomes mixed with the ink near and at the plate surface and returns to the inking train of rollers and may ultimately be introduced into the ink supply system itself. In any case, these conventional lithographic systems require considerable operator attention to maintain ink and dampening solution balance and produce more product waste than desired.
Prior art devices and methods for correcting this inherent fault in conventional lithography utilize keyless inkers. Certain of these methods also involve eliminating the dampening system or eliminating operator control of the dampening system.
Keyless inking systems have been disclosed that purport to eliminate operator attention to column control of inking by elimination of adjustable inking keys, thereby avoiding many of the aforementioned disadvantages of conventional lithography. For keyless inking systems an ink metering method is required that continues to function despite the presence of up to about 40% dampening solution in the ink without allowing any temporarily-free dampening solution to interfere with the ink-metering function. Also, the unused or non-uniform portion of the initially uniform ink film that is being continuously presented to the printing plate must be continuously scraped-off the return side of the inking system to enable continuous presentation of the uniform ink film to the plate by the supply side of the inking system. This scraped-off film is not uniform across the width of the press in ink and dampening solution composition. Since it would not be economically feasible to continuously discard the ink in the unused portion of the ink and dampening solution mixture, this mixture must either be renewed by selectively removing dampening solution from the mixture and returning the ink portion to the inking system or by thoroughly intermixing the unused ink and dampening solution mixture with fresh replenishment ink and returning such mixture to the inking system. U.S. Pat. No. 4,690,055 discloses a keyless inking system in which dampening solution removal is unnecessary and which accommodates the dampening solution that is naturally acquired in the unused ink during the practice of lithography and for which, therefore, removal of dampening solution is not required.
In the keyless inking system disclosed in U.S. Pat. No. 4,690,055 (hereby incorporated by reference), the location of the dampening system is not critical and can be positioned either to supply dampening solution directly to the plate cylinder or at some other location such as at an oscillator drum to which ink is also being supplied. An ink circulating and mixing system receives new or replenishment ink, as well as, the ink and dampening solution combination, that is continuously returned from a doctor blade which scrapes excess printing fluid from a rotating metering roller. Such ink and dampening combinations are generally herein referred to as printing fluids. The printing fluid circulating and mixing system functions to assure an inherently uniform cross-press input of printing fluid that remains consistent throughout a printing run and consists of a printing fluid pan roller, pump and appropriate conduits, a printing fluid pan level controlling system, and a printing fluid reservoir of such volume and design that it assures the printing fluid being fed to the metering roller is uniform in composition at any given instant of time despite the existence of the continual cross-press dampening solution to ink ratio differences of the unused or scraped return printing fluid previously referred to. The printing fluid circulation system is designed to continuously collect and distribute the printing fluid from a reservoir through a plenum or series of orifices to uniformly redistribute the printing fluid across the press width to provide uniform composition of the printing fluid that is being introduced to a celled metering roller. The metering roller can be one of the types shown and described in U.S. Pat. Nos. 4,882,990, 4,537,127, 4,862,799, 4,567,827, or 4,601,242, (all of which are hereby incorporated by reference) or any wear resistant oleophilic and hydrophobic metering roller as substantially therein defined.
Although the system disclosed in U.S. Pat. No. 4,690,055 provides great improvements in lithographic printing presses, the technology provides only a fixed volume of ink input. In this prior art system the return ink film is removed in sequence after the metering roller surface has been refilled with replacement printing fluid by means of a single doctoring blade. Other prior art systems use two doctor blades, the first in sequence removing the return unused ink film, the second next in sequence removing excess refill ink that had been purposefully applied to assure complete filling of the metering roller cells. None of these prior art inking systems have proven means for purposefully varying the amount of ink being metered into the press in a manner that is uniform across the width of the press.
With flexographic or gravure keyless inking system which use highly fluid inks, pigment content in the ink can readily be varied at press-side to accomplish the effect of delivering more or less coloration (pigment) to the substrate being printed. When using viscous oil-based lithographic inks, press-side alteration of the ink is generally not an acceptable alternative for practical operational reasons.
Changing to a metering roller having larger or smaller ink delivery capacity is another alternative for changing the ink input quantity and therefore the pigment delivery quantity, which therefore changes the printed optical density. This requires designing the press with quick-roller change capability as a criterion, often at the sacrifice of other machine or operational design options. Also, the metering rollers for large, high-speed presses are heavy, requiring mechanical lifting assistance devices. Such changes are generally not sufficiently rapid for use in high-speed, high volume printing operations. Means are needed to avoid these impractical means for modulation of keyless inking printed optical density values.
The present invention overcomes the problems, difficulties and inconveniences associated with fixed volume ink input systems, yet retains all of the principles essential to keyless lithographic systems as disclosed in U.S. Pat. No. 4,690,055. Accordingly, in this improvement a variable thickness ink or printing fluid film is metered past a doctor blade or equivalent structure. A separate device is provided for removing the return ink film.